1. Field of the Invention
This invention relates to a filter device and a radio communication terminal equipment, and more particularly, is applicable to a filter device and a radio communication terminal equipment which are employed in a radio communication system in which the communication is performed through such a communication medium that the available frequency band is restricted.
2. Description of the Related Art
Heretofore, in a radio communication terminal equipment of this kind, the data signal which is to be transmitted (this is referred to as the base band signal, hereinafter) is generated as a digital signal. The radio communication terminal equipment converts the generated base-band signal into an analog signal which stays within the stated frequency band by the use of a filter device. After that, a carrier wave is modulated with the base band signal which has been converted into the analog signal and the modulated signal is transmitted.
FIG. 1 generally shows a circuit configuration of a filter device which has been installed within a radio communication terminal equipment, and a clock signal S1 which has been generated in an oscillator 1 is sent to a frequency divider 2. The frequency divider 2 lowers the frequency of the clock signal S1 into one over an integer, so as to generate a reference clock signal S2, and then supplies it to a frequency divider 3 and also supplies it to a transmission filter 4. The frequency divider 3 further lowers the reference clock signal S2 into one over an integer, so as to generate a reference clock signal S3, and then supplies it to a transmission symbol generating division 5. This radio communication terminal is used as a terminal which is directed to CDMA-PCS, and J-STD-008 of ANSI standard prescribes that the frequency of the base band signal which is generated in the transmission symbol generating division 5 should be 1.2288 MHz. Therefore, the clock signal S1 which is generated in the oscillator 1 should be 19.6608 MHz, the reference clock signal S2 which is generated in the frequency divider 2 should be 4.9152 MHz, and the reference clock signal S3 which is generated in the frequency divider 3 should be 1.2288 MHz.
The transmission symbol generating division 5 generates a base band signal S4, on the basis of the given reference clock signal S3. The transmission symbol generating division 5 sends the base band signal S4 which has the same frequency as that of the reference clock signal S3 to the transmission filter 4.
The transmission filter 4 is comprised of a re-sampler 6, a digital filter 7, and a digital-to-analog converter (this is referred to as a D/A converter) 8, and acts on the basis of the reference clock signal S2 which is given from the frequency divider 2. The transmission filter 4 inputs the base band signal S4 which is given from the transmission symbol generating division 5 to the re-sampler 6. The re-sampler 6 performs over-sampling processing of the base band signal S4 with the frequency of the reference clock signal S2. The re-sampler 6 thus performs the over-sampling processing on the basis of the reference clock signal S2, the frequency of which is an integer times that of the base band signal S4. The re-sampler 6 delivers a pulse signal S5 which has been obtained through such over-sampling processing to the digital filter 7. The digital filter 7 re-quantizes the pulse signal S5, in accordance with the stated frequency characteristic. The digital filter 7 delivers a quantized signal S6 which has been obtained by such quantization to the D/A converter 8. The D/A converter 8 converts the quantized signal S6 into an analog signal, and then extracts only the stated frequency band of this with an analog low-pass filter (not shown) and outputs it.
That is, as shown in FIG. 2A, the base band signal S4 which has been produced in the transmission symbol generating division 5 is an impulse string of 1.2288 MHz, and, when this is viewed on the frequency axis, a band of 1.2288 MHz width appears repeatedly for each 1.2288 MHz.
As shown in FIG. 2B, the re-sampler 6 performs over-sampling of such base band signal S4 with 4.9152 MHz, which is four times 1.2288 MHz, so as to broaden one bandwidth of the base band signal S4 into 4.9152 MHz width. This one band of 4.9152 MHz contains the original information of 1.2288 MHz as much as four times over. To be concrete, such over-sampling sampling is achieved by interpolating the information of three "0" (dotted portions between the respective impulses, in the figure) between the impulse string of the base band signal S4.
As shown in FIG. 2C, the band of the pulse signal S5 which has been thus obtained by over-sampling is narrowed by the digital filter 7. By sending the quantized signal S6 which has been obtained by narrowing the bandwidth in this way, the transmission filter 4 is able to issue the signal which forms the envelope shown in the figure. By outputting the thus issued signal via an analog low-pass filter, the radio communication terminal equipment extracts and outputs only the stated frequency band.
By performing over-sampling processing in this way with the transmission filter 4, the radio communication terminal equipment is able to broaden the respective intervals between the frequency bands of the base band signal S4, and is so able to extract the stated frequency band easily with the analog low-pass filter (not shown). That is, the radio communication terminal equipment is adapted to alleviate the characteristics which are required of the analog low-pass filter so as to reduce the load, by exposing the base band signal S4 to over-sampling processing.
On the other hand, J-STD-008 of the above-mentioned ANSI standard prescribes that the setting interval of the code division multiple access (CDMA) channels which are used for transmission/reception should be 50 kHz. So, in a radio communication terminal equipment based on the standard, a clock signal of an integer times 50 kHz is generated, and this is supplied to a frequency divider, which has been equipped within an RF module for performing frequency adjustment of the transmitted/received signal, and a PLL synthesizer is activated, and thereby tuning is performed in a 50 kHz division manner.
That is, from the above, in a radio communication terminal equipment based on J-STD 008 of ANSI standard, it is required to equip oscillators each generating a clock signal for generating the base band signal S4 and a clock signal for performing channel tuning, separately. At here, it may be suggested to provide a oscillator which generates a clock signal having a frequency of the common multiple of the both, however, it is at variance with the real conditions because the frequency becomes very high.
By the way, in the radio communication terminal equipment of such configuration, both the oscillator 1 for generating the base band signal S4 and an oscillator for tuning of the channels are needed, as stated above. In this connection, an oscillator which is referred to as a voltage controlled temperature compensated quartz-crystal oscillator (VCTCXO) is usually utilized as an oscillator for generating the clock signal which is used for the radio communication terminal equipment, because the clock signal requires high precision.
However, in the case where separate oscillators are provided in this way, there are such problems that the locations of the respective oscillators are set in the equipment and so the mounting area for mounting of the other components is reduced, so that the configuration may be complicated and miniaturization of the equipment may be interrupted. To avoid such problems, an approach is needed wherein clock signals of two frequencies are respectively generated from one oscillator.
For instance, a clock signal untouched which has been generated in the oscillator is used as the clock signal for channel tuning. On the other hand, frequency conversion of the clock signal which has been generated by the oscillator is performed to produce the clock signal for generating the base band signal. In this way, the clock signals having two different frequencies can be obtained from the clock signal which is generated by one oscillator.
However, the clock signal for base-band signal creation which has been obtained by such frequency conversion contains jitter components, because it is not an oscillated signal which has been generated by an oscillator. Such a clock signal does raise a large problem at the time of creation of the base band signal of digital signal. But, there is such a problem that a spectral distortion occurs in the outputted transmission signal, in the case where the base band signal which has been produced on the basis of such a clock signal is subjected to digital-to-analog conversion and then inputted to the transmission filter for creating the analog transmission signal.